Structure of the glacial thermocline at Little Bahama Bank

Abstract

Within the thermocline of the western North Atlantic, bathymetric gradients of hydrographic properties are controlled by the lateral movement of water along isopycnal surfaces^1–4. This movement is driven by air–sea interaction processes :Ekman pumping and sur-face heat (buoyancy) flux. By affecting the intensity of these processes, glacial7ndash;interglacial climatic change should also affect the bathymetric gradients of properties within the thermocline. Benthic foraminifera live on carbonate bank margins that intersect the thermocline. These foraminifera record the hydrographic gradients in their isotopic shell chemistry, providing the means to reconstruct changes in thermocline structure and circulation. We have measured the δ¹⁸O compositions of a near-surface planktonic foraminifer, Globigerinoides ruber, and a shallow-water benthic foraminifer, Cibicidoides floridanus, in a core recovered from Little Bahama Bank. Cibicidoides floridanus exhibits a glacial–inter-glacial range in δ¹⁸O that is 0.5% less than G. ruber's, because individuals of this species lived and calcified ~110m shallower on the thermocline ~ 18,000 yr ago. After glacio-eustatic changes in thermocline position are accommodated, we estimate that temperature at 540 m palaeodepth cooled by ~ 2 ° C. Globigerionoides ruber δ¹⁸O values suggest that surface water temperature at this location also cooled by ~2 °C. This uniform cooling of the upper 600 m of the water column is consistent with the observation that surface water where isopycnals outcrop was ~2 °C cooler during the glacial maximum^5–8.